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Safety in numbers: the dilution effect and other drivers of group life in the face of danger

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Abstract

Animals can congregate in groups for many reasons, from reproductive assurance to improved foraging or predation efficiency, to avoiding themselves becoming the target of predation by other animals. It is the last category that is the focus of this review: group living as protection from predation. The drivers of group life in the face of danger are at the same time diverse and interlinked, with much potential for confusion between concepts. Here we review these concepts, using the dilution effect as a starting point. We construct a mathematical model that allows us to examine various features of the dilution effect and their connection to ecology. We also show the importance of including a time scale when modelling the dilution effect and how this translates into more realistic estimation of the fitness consequences of a diluted predation risk. The central role of the dilution effect in creating safety in numbers is underlined by showing how it may affect life-history evolution and result in the emergence of gregarious life-history strategies, even among sessile organisms limited in their abilities to exhibit behavioural responses to predation. Finally, we review the other central processes underpinning group protection from predation: the satiation effect, selfish herding, the confusion effect and group vigilance.

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References

  • Ballerini M, Cabibbo N, Candelier R et al (2008) Empirical investigation of starling flocks: a benchmark study in collective animal behaviour. Anim Behav 76:201–215

    Article  Google Scholar 

  • Beauchamp G (2003) Group-size effects on vigilance: a search for mechanisms. Behav Proc 63:111–121

    Article  Google Scholar 

  • Beauchamp G, Ruxton GD (2003) Changes in vigilance with group size under scramble competition. Am Nat 161:672–675

    Article  CAS  PubMed  Google Scholar 

  • Bednekoff PA, Lima SL (1998) Re-examining safety in numbers: interactions between risk dilution and collective detection depend upon predator targeting behaviour. Proc R Soc Lond B 265:2021–2026

    Article  Google Scholar 

  • Bednekoff PA, Lima SL (2004) Risk allocation and competition in foraging groups: reversed effects of competition if group size varies under risk of predation. Proc R Soc Lond B 271:1491–1496

    Article  Google Scholar 

  • Charlesworth B (1994) Evolution in age-structured populations, 2nd edn. Cambridge University Press, Cambridge

    Book  Google Scholar 

  • Charnov EL, Krebs JR (1975) The evolution of alarm calls: altruism or manipulation? Am Nat 109:107–112

    Article  Google Scholar 

  • Clark CW, Mangel M (1986) The evolutionary advantages of group foraging. Theor Pop Biol 30:45–75

    Article  Google Scholar 

  • Cockrem JF, Silverin B (2002) Sight of a predator can stimulate a corticosterone response in the great tit (Parus major). Gen Comp Endocrinol 125:248–255

    Article  CAS  PubMed  Google Scholar 

  • Courchamp F, Clutton-Brock T, Grenfell B (1999) Inverse density dependence and the Allee effect. Trends Ecol Evol 14:405–410

    Article  PubMed  Google Scholar 

  • Couzin ID, Krause J, James R, Ruxton GD, Franks NR (2002) Collective memory and spatial sorting in animal groups. J Theor Biol 218:1–11

    Article  PubMed  Google Scholar 

  • Cresswell W (1994) Flocking is an effective anti-predation strategy in redshanks, Tringa totanus. Anim Behav 47:433–442

    Article  Google Scholar 

  • Cresswell W, Quinn JL (2011) Predicting the optimal prey group size from predator hunting behaviour. J Anim Ecol 80:310–319

    Article  PubMed  Google Scholar 

  • Curley EA, Rowley HE, Speed MP (2015) A field demonstration of the costs and benefits of group living to edible and defended prey. Biol Lett 11:20150152

  • Dale BW, Adams LG, Bowyer RT (1994) Functional response of wolves preying on barren-ground caribou in a multiple-prey ecosystem. J Anim Ecol 63:644–652

    Article  Google Scholar 

  • Daly D, Higginson AD, Chen D, Ruxton GD, Speed MP (2012) Density-dependent investment in costly anti-predator defences: an explanation for the weak survival benefit of group living. Ecol Lett 15:576–583

    Article  PubMed  Google Scholar 

  • Ekman I (1987) Exposure and time use in willow tit flocks: the cost of subordination. Anim Behav 35:445–452

    Article  Google Scholar 

  • Eshel I, Shaked A (2001) Partnership. J Theor Biol 208:457–474

    Article  CAS  PubMed  Google Scholar 

  • Finkbeiner SD, Briscoe AD, Reed RD (2012) The benefit of being a social butterfly: communal roosting deters predation. Proc R Soc Lond B 279:2769–2776

    Article  Google Scholar 

  • Fischer B, Taborsky B, Kokko H (2011) How to balance the offspring quality–quantity tradeoff when environmental cues are unreliable. Oikos 120:258–270

    Article  Google Scholar 

  • Fletcher QE, Boutin S, Lane JE, LaMontagne JM, McAdam AG, Krebs CJ, Humphries MM (2010) The functional response of a hoarding seed predator to mast seeding. Ecology 91:2673–2683

    Article  PubMed  Google Scholar 

  • Fordyce JA, Agrawal AA (2001) The role of plant trichomes and caterpillar group size on growth and defence of the pipevine swallowtail Battus philenor. J Anim Ecol 70:997–1005

    Article  Google Scholar 

  • Gloag R, Fiorini VD, Reboreda JC, Kacelnik A (2012) Brood parasite eggs enhance egg survivorship in a multiply parasitized host. Proc R Soc Lond B 279:1831–1839

    Article  Google Scholar 

  • Godin J-GJ, Classon LJ, Abrahams MV (1988) Group vigilance and shoal size in a small characin fish. Behaviour 104:29–40

    Article  Google Scholar 

  • Hamilton WD (1964) The genetical evolution of social behavior. I & II. J Theor Biol 7:1–52

    Article  CAS  PubMed  Google Scholar 

  • Hamilton WD (1971) Geometry for the selfish herd. J Theor Biol 31:295–311

    Article  CAS  PubMed  Google Scholar 

  • Hawlena D, Schmitz OJ (2010) Physiological stress as a fundamental mechanism linking predation to ecosystem functioning. Am Nat 175:537–556

    Article  Google Scholar 

  • Holling CS (1959) Some characteristics of simple types of predation and parasitism. Can Entomol 91:385–398

    Article  Google Scholar 

  • Jaatinen K, Öst M (2013) Brood size matching: a novel perspective on predator dilution. Am Nat 181:171–181

    Article  PubMed  Google Scholar 

  • Jaatinen K, Öst M, Waldeck P, Andersson M (2009) Clutch desertion in Barrow’s goldeneyes (Bucephala islandica)—effects of non-natal eggs, the environment and host female characteristics. Ann Zool Fenn 46:350–360

    Article  Google Scholar 

  • Jaatinen K, Lehtonen J, Kokko H (2010) Strategy selection under conspecific brood parasitism: an integrative modeling approach. Behav Ecol 22:144–155

    Article  Google Scholar 

  • Jaatinen K, Öst M, Lehikoinen A (2011) Adult predation risk drives shifts in parental care strategies: a long-term study. J Anim Ecol 80:49–56

    Article  PubMed  Google Scholar 

  • Jordan LA, Avolio C, Herbert-Read JE, Krause J, Rubenstein DI, Ward AJ (2010) Group structure in a restricted entry system is mediated by both resident and joiner preferences. Behav Ecol Sociobiol 64:1099–1106

    Article  Google Scholar 

  • Karban R (1982) Increased reproductive success at high densities and predator satiation for periodical cicadas. Ecology 63:321–328

    Article  Google Scholar 

  • Kelly D, Sork VL (2002) Mast seeding in perennial plants: why, how, where? Ann Rev Ecol Syst 33:427–447

    Article  Google Scholar 

  • Kingman JF (1993) Poisson processes. Oxford studies in probability, vol 3. Oxford University Press, Oxford

    Google Scholar 

  • Koenig WD, Kelly D, Sork VL, Duncan RP, Elkinton JS, Peltonen MS, Westfall RD (2003) Dissecting components of population-level variation in seed production and the evolution of masting behavior. Oikos 102:581–591

    Article  Google Scholar 

  • Kokko H, Rankin DJ (2006) Lonely hearts or sex in the city? Density-dependent effects in mating systems. Philos T Roy Soc B 361:319–334

    Article  Google Scholar 

  • Kokko H, Johnstone RA, Clutton-Brock TH (2001) The evolution of cooperative breeding through group augmentation. Proc R Soc Lond B 268:187–196

    Article  CAS  Google Scholar 

  • Krakauer DC (1995) Groups confuse predators by exploiting perceptual bottlenecks: a connectionist model of the confusion effect. Behav Ecol Sociobiol 36:421–429

    Article  Google Scholar 

  • Krause J, Godin J-GJ (1995) Predator preferences for attacking particular prey group sizes: consequences for predator hunting success and prey predation risk. Anim Behav 50:465–473

    Article  Google Scholar 

  • Krause J, Ruxton GD (2002) Living in groups. Oxford University Press, Oxford

    Google Scholar 

  • Krause J, Ruxton GD, Rubenstein D (1998) Is there always an influence of shoal size on predator hunting success? J Fish Biol 52:494–501

    Article  Google Scholar 

  • Kruuk H (1972) The spotted hyena: a study of predation and social behavior. University of Chicago Press, Chicago

    Google Scholar 

  • LaGory KE (1986) Habitat, group size, and the behaviour of white-tailed deer. Behaviour 98:168–179

    Article  Google Scholar 

  • Le Masurier AD (1994) Costs and benefits of egg clustering in Pieris brassicae. J Anim Ecol 63:677–685

    Article  Google Scholar 

  • Lengyel S (2007) Benefits of large broods by higher chick survival and better territories in a precocial shorebird. Behav Ecol Sociobiol 61:589–598

    Article  Google Scholar 

  • Lima SL (1990) The influence of models interpretation of vigilance. In: Bekoff M, Jamieson D (eds) Interpretation and explanation in the study of animal behavior, explanation, evolution and adaption, vol 2. Westview press, Boulder, pp 246–267

    Google Scholar 

  • Lima S (1995) Back to the basics of anti-predatory vigilance: the group-size effect. Anim Behav 49:11–20

    Article  Google Scholar 

  • Lima SL (2002) Putting predators back into behavioral predator–prey interactions. Trends Ecol Evol 17:70–75

    Article  Google Scholar 

  • Lima SL, Bednekoff PA (1999) Temporal variation in danger drives antipredator behavior: the predation risk allocation hypothesis. Am Nat 153:649–659

    Article  Google Scholar 

  • Lindstedt C, Mappes J, Paivinen J, Varama M (2006) Effects of group size and pine defence chemicals on Diprionid sawfly survival against ant predation. Oecologia 150:519–526

    Article  PubMed  Google Scholar 

  • Lindstedt C, Huttunen H, Kakko M, Mappes J (2011) Disentangling the evolution of weak warning signals: high detection risk and low production costs of chemical defences in gregarious pine sawfly larvae. Evol Ecol 25:1029–1046

    Article  Google Scholar 

  • Maynard Smith J (1965) The evolution of alarm calls. Am Nat 99:59–63

    Article  Google Scholar 

  • Miller R (1922) The significance of the gregarious habit. Ecology 3:122–126

    Article  Google Scholar 

  • Mooring MS, Hart BL (1992) Animal grouping for protection from parasites: selfish herd and encounter-dilution effects. Behaviour 123:173–193

    Article  Google Scholar 

  • Morgan MJ, Godin J-GJ (1985) Antipredator benefits of schooling behaviour in a cyprinodontid fish, the banded killifish (Fundulus diaphanus). Z Tierpsychol 70:236–246

    Article  Google Scholar 

  • Newton I (1994) The role of nest sites in limiting the numbers of hole-nesting birds: a review. Biol Conserv 70:265–276

    Article  Google Scholar 

  • Nøttestad L, Axelsen BE (1999) Herring schooling manoeuvres in response to killer whale attacks. Can J Zool 77:1540–1546

    Article  Google Scholar 

  • Öst M, Mantila L, Kilpi M (2002) Shared care provides time-budgeting advantages for female eiders. Anim Behav 64:223–231

    Article  Google Scholar 

  • Otto SP, Day T (2007) A biologist’s guide to mathematical modeling in ecology and evolution. Princeton University Press, Princeton

    Google Scholar 

  • Packer C, Gilbert DA, Pusey AE, O’Brien SJ (1991) A molecular genetic analysis of kinship and cooperation in African lions. Nature 351:562–565

    Article  CAS  Google Scholar 

  • Pitcher TJ, Partridge BL (1979) Fish school density and volume. Mar Biol 54:383–394

    Article  Google Scholar 

  • Pulliam HR (1973) On the advantages of flocking. J Theor Biol 38:419–422

    Article  CAS  PubMed  Google Scholar 

  • Pulliam HR, Pyke GH, Caraco T (1982) The scanning behavior of juncos: a game-theoretical approach. J Theor Biol 95:89–103

    Article  Google Scholar 

  • Ricklefs RE, Wikelski M (2002) The physiology/life-history nexus. Trends Ecol Evol 17:462–468

    Article  Google Scholar 

  • Riipi M, Alatalo RV, Lindström L, Mappes J (2001) Multiple benefits of gregariousness cover detectability costs in aposematic aggregations. Nature 413:512–514

    Article  CAS  PubMed  Google Scholar 

  • Roberts G (2005) Cooperation through interdependence. Anim Behav 70:901–908

    Article  Google Scholar 

  • Rode NO, Lievens EJ, Flaven E, Segard A, Jabbour-Zahab R, Sanchez MI, Lenormand T (2013) Why join groups? Lessons from parasite-manipulated Artemia. Ecol Lett 16:493–501

    Article  PubMed  Google Scholar 

  • Roy Nielsen CL, Parker PG, Gates RJ (2008) Partial clutch predation, dilution of predation risk, and the evolution of intraspecific nest parasitism. Auk 125:679–686

    Article  Google Scholar 

  • Rubenstein DI (1978) On predation, competition, and the advantages of group living. Persp Ethol 3:205–231

    Article  Google Scholar 

  • Scheuerlein A, Van’t Hof T, Gwinner E (2001) Predators as stressors? Physiological and reproductive consequences of predation risk in tropical stonechats (Saxicola torquata axillaris). Proc R Soc Lond B 270:799–803

    Google Scholar 

  • Sillén-Tullberg B, Leimar O (1988) The evolution of gregariousness in distasteful insects as a defense against predators. Am Nat 132:723–734

    Article  Google Scholar 

  • Solomon ME (1949) The natural control of animal populations. J Anim Ecol 18:1–35

    Article  Google Scholar 

  • Stearns (1992) The evolution of life histories. Oxford University Press, Oxford

    Google Scholar 

  • Stephens PA, Russell AF, Young AJ, Sutherland WJ, Clutton-Brock TH (2005) Dispersal, eviction, and conflict in meerkats (Suricata suricatta): an evolutionarily stable strategy model. Am Nat 165:120–135

    Article  CAS  PubMed  Google Scholar 

  • Sweeney BW, Vannote RL (1982) Population synchrony in mayflies: a predator satiation hypothesis. Evolution 36:810–821

    Article  Google Scholar 

  • Symington MM (1988) Food competition and foraging party size in the black spider monkey (Ateles paniscus Chamek). Behaviour 105:117–132

    Article  Google Scholar 

  • Treherne JE, Foster WA (1980) The effects of group size on predator avoidance in a marine insect. Anim Behav 28:1119–1122

    Article  Google Scholar 

  • Treherne JE, Foster WA (1982) Group size and anti-predator strategies in a marine insect. Anim Behav 30:536–542

    Article  Google Scholar 

  • Tucker JK, Paukstis GL, Janzen FJ (2008) Does predator swamping promote synchronous emergence of turtle hatchlings among nests? Behav Ecol 19:35–40

    Article  Google Scholar 

  • Turner GF, Pitcher TJ (1986) Attack abatement: a model for group protection by combined avoidance and dilution. Am Nat 128:228–240

    Article  Google Scholar 

  • Wheeler BC (2008) Selfish or altruistic? An analysis of alarm call function in wild capuchin monkeys, Cebus apella nigritus. Anim Behav 76:1465–1475

    Article  Google Scholar 

  • Whitfield DP (2003) Redshank Tringa totanus flocking behaviour, distance from cover and vulnerability to sparrowhawk Accipiter nisus predation. J Avian Biol 34:163–169

    Article  Google Scholar 

  • Wrona FJ, Dixon RJ (1991) Group size and predation risk: a field analysis of encounter and dilution effects. Am Nat 137:186–201

    Article  Google Scholar 

  • Yano S (2012) Cooperative web sharing against predators promotes group living in spider mites. Behav Ecol Sociobiol 66:845–853

    Article  Google Scholar 

  • Zöttl M, Frommen JG, Taborsky M (2013) Group size adjustment to ecological demand in a cooperative breeder. Proc R Soc B 280:20122772

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Acknowledgments

We are grateful to Hanna Kokko for the many discussions on the topic and for the constructive feedback on early versions of the manuscript. Two anonymous reviewers provided further comments which significantly improved this work. JL was funded by the Kone Foundation and a University of New South Wales Vice-Chancellor’s Postdoctoral Research Fellowship. KJ was funded by the Academy of Finland (grant number 266208) and the Finnish Cultural foundation.

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    Authors and Affiliations

    1. Evolution and Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales, 2052, Australia

      Jussi Lehtonen

    2. ARONIA Coastal Zone Research Team, Novia University of Applied Sciences, Raseborgsvägen 9, FI-10600, Ekenäs, Finland

      Kim Jaatinen

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    1. Jussi Lehtonen
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    Correspondence to Kim Jaatinen.

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    Communicated by P. M. Kappeler

    Jussi Lehtonen and Kim Jaatinen contributed equally to this work.

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    Lehtonen, J., Jaatinen, K. Safety in numbers: the dilution effect and other drivers of group life in the face of danger. Behav Ecol Sociobiol 70, 449–458 (2016). https://doi.org/10.1007/s00265-016-2075-5

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    • DOI: https://doi.org/10.1007/s00265-016-2075-5

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